Structure and method for index-guided buried heterostructure algalnn laser diodes

ABSTRACT

An index-guided buried heterostructure AlGaInN laser diode provides improved mode stability and low threshold current when compared to conventional ridge waveguide structures. A short period superlattice is used to allow adequate cladding layer thickness for confinement without cracking. The intensity of the light lost due to leakage is reduced by about 2 orders of magnitude with an accompanying improvement in the far-field radiation pattern when compared to conventional structures.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to commonly assigned, concurrentlyfiled Bour et al. U.S. patent application entitled “STRUCTURE AND METHODFOR SELF-ALIGNED, INDEX-GUIDED BURIED HETEROSTRUCTURE AlGaInN LASERDIODES” (application Ser. No. ______, Attorney Reference No. D/99241)which is included by reference in its entirety.

FIELD OF INVENTION

[0002] This invention relates to nitride based blue laser diodes.

BACKGROUND OF INVENTION

[0003] Nitride based blue laser diodes are being developed for printingand optical data storage applications. The first AlGaInN laser diodeswere broad area lasers providing no control over the laser diode'svarious spatial modes. Most applications, however, require the laserdiode to operate in a single spatial mode. One way of achieving singlespatial mode operation for AlGaIN blue laser diodes is to use a ridgewaveguide structure to define a lateral waveguide as described in“Ridge-geometry InGaN multi-quantum-well-structure laser diodes” by S.Nakamura et al., in Applied Physics Letters 69 (10), pp. 1477-1479 whichis hereby incorporated by reference in its entirety. While a ridgewaveguide provides for single spatial mode emission in blue lasers, thewaveguiding provided is relatively weak. The lateral refractive indexstep is small and is influenced by heating and carrier injection.Additionally, there are fabrication difficulties because the ridge mustbe etched to extend sufficiently close to the laser active regionwithout the ability to use an etch stop to prevent material damage tothe laser active region since chemical etching is not applicable to GaNmaterials.

[0004] To provide stronger mode stability and low threshold currentoperation, more strongly index-guided diode lasers are required such asthose having buried heterostructures that are typically used for InGaAsPfiber optic-communication lasers, or theimpurity-induced-layer-disordered waveguide structures used forhigh-power single-mode AlGaAs laser diodes. Additionally, the use of aburied heterostructure avoids certain fabrication difficulties.

BRIEF SUMMARY OF INVENTION

[0005] Both index-guided buried heterostructure AlGaInN laser diodes andself-aligned index guided buried heterostructure AlGaInN laser diodesprovide improved mode stability and low threshold current when comparedto conventional ridge waveguide structures. A structure for theindex-guided buried heterostructure AlGaInN laser diode in accordancewith the invention typically uses insulating AlN, AlGaN or p-dopedAlGaN:Mg for lateral confinement and has a narrow (typically about 1-5μm in width) ridge which is the location of the narrow active stripe ofthe laser diode which is defined atop the ridge. The narrow ridge issurrounded by an epitaxially deposited film having a window on top ofthe ridge for the p-electrode contact. The ridge is etched completelythrough the active region of the laser diode structure to the shortperiod superlattice n-cladding layer. Typically, use of a short periodsuperlattice allows doubling of the cladding layer thickness withoutcracking. This reduces the intensity of the light lost due to leakage byabout 2 orders of magnitude with an accompanying improvement in thefar-field radiation pattern in comparison with conventional structures.Junction surfaces are exposed by the ridge etch and these junctionsurfaces contribute surface states which prevent injected carriers fromfilling conduction or valence band states needed for a populationinversion. However, the epitaxial regrowth of a high bandgap materialpassivates the surface states because the interface between theovergrown material and the ridge structure is perfectly coherent.

[0006] The structure for the self-aligned, index guided, buriedheterostructure AlGaInN laser diode uses the p-cladding layer to alsofunction as the burying layer to provide strong lateral opticalconfinement and strong lateral carrier confinement. The p-claddinglayer/burying layer is typically AlGaN:Mg. The structure for theself-aligned, index guided, buried heterostructure laser diode issimpler than for the index-guided, buried heterostructure AlGaInN laserdiode. The laser structure is grown through the active quantum well andwaveguide region followed by etching a narrow laser ridge down to then-bulk cladding layer. The p-type cladding/burying layer is thenovergrown around the ridge along with the p-contact layer. Subsequentlaser processing is simple since the two-step growth process results ina lateral waveguide and carrier confinement structure which does notrequire the creation of contact windows. Hence, the laser processingrequired is basically a broad area laser fabrication sequence.Additionally, the comparatively large p-contact area allowed by theself-aligned architecture contributes to a lower diode voltage and lessheat during continuous wave operation of the laser diode.

[0007] The advantages and objects of the present invention will becomeapparent to those skilled in the art from the following detaileddescription of the invention, its preferred embodiments, theaccompanying drawings which are illustrative and not to scale, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows an embodiment of an index guided, buriedheterostructure laser diode structure in accordance with the invention.

[0009]FIG. 2 shows an embodiment of an index guided, buriedheterostructure laser diode structure in accordance with the invention.

[0010]FIG. 3 shows an embodiment of a self-aligned, index guided, buriedheterostructure laser diode in accordance with the invention.

[0011]FIG. 4 shows the carrier paths for the embodiment shown in FIG. 3

[0012]FIG. 5 shows an embodiment of a self-aligned, index guided, buriedheterostructure laser diode in accordance with the invention.

[0013] FIGS. 6-11 show process steps for making a self-aligned, indexguided, buried heterostructure laser diode in accordance with theinvention.

DETAILED DESCRIPTION

[0014]FIG. 1 shows index-guided, buried heterostructure AlGaInN laserdiode structure 100 in accordance with the present invention. GaN:Silayer 115 is positioned on Al₂O₃ growth substrate 110 and in oneembodiment layer 115 may be made of AlGaN:Si to reduce optical leakage.Short period superlattice n-cladding structure 121, typically made up ofalternating layers of A1 _(0.15)Ga_(0.85)N:Si and GaN:Si each with atypical thickness of about 20 Å, is positioned below the GaN n-waveguidelayer (not shown in FIG. 1) at the bottom of InGaN multiple quantum wellstructure 145. Introduction of short period superlattice n-claddingstructure 121 allows increased cladding thickness to significantlyreduce leakage of the transverse optical mode and results in an improvedtransverse far-field pattern for laser diode structure 100. For example,a typical leakage of about 7% may be reduced to 0.5% The far-field beampattern approaches a Gaussian far-field beam.

[0015] P-cladding layer 125, typically Al_(0.07)Ga_(0.93)N:Mg, ispositioned over the GaN p-waveguide layer (not shown in FIG. 1) which isadjacent to the tunnel barrier layer (not shown in FIG. 1), typicallyAl_(0.2)Ga_(0.8)N:Mg, present at the top of InGaN multiple quantum wellstructure 145. Layer 185 serves as a capping layer to facilitate ohmiccontact. Burying layer 155 is positioned over capping layer 185,typically GaN:Mg, with windows through burying layer 155 to allowp-electrode 190 to contact GaN:Mg layer 185 and n-electrode 195 tocontact GaN:Si layer 115.

[0016] Burying layer 155, typically insulating AlN or AlGaN, has a lowrefractive index which results in strong lateral index guiding becausethe refractive index step is typically around 0.1. With such a largelateral index step, the lateral waveguiding in index-guided, buriedheterostructure AlGaInN laser diode structure 100 overwhelms thermal orcarrier injection influences to provide a more stable and lessastigmatic beam pattern. Burying layer 155 also has a high bandgapenergy which results in high lateral carrier confinement.

[0017] Undoped AlN films are insulating and prevent formation of a shuntpath around InGaN multiple quantum well structure 145. In one embodimentof index-guided, buried heterostructure AlGaInN laser diode structure100 in accordance with the invention, buried layer 155 is an AlGaN:Mgdoped layer as undoped AlGaN may not be insulating depending on growthconditions and the aluminum content. The optoelectronic character ofAlGaN depends on growth conditions. For example, it is possible to growinsulating GaN at low temperatures (approximately 900° C.), while athigher growth temperatures GaN tends to have an n-type backgroundconductivity. The precise mechanism for the n-conductivity is presumedto arise from either native defects and or impurities. Oxygen andsilicon are both commonly encountered shallow, unintentional donors inGaN. Low aluminum-content AlGaN that is magnesium doped behavessimilarly to GaN except that the magnesium acceptor's activation energyincreases at the rate of about 3 meV for each percent aluminum added inthe alloy up to a 20 percent aluminum content. For aluminum contentabove 20 percent, unintentional oxygen incorporation may result inuncontrollably high n-type background conductivity. Oxygen is readilyincorporated into AlGaN because of the high affinity of aluminum foroxygen and oxygen impurities are typically available from varioussources during MOCVD growth. While oxygen impurities may be compensatedby magnesium acceptors this is difficult in practice and suggests itwould be difficult to make high aluminum content AlGaN burying layersthat are insulating. High aluminum content provides better optical andcarrier confinement.

[0018] Due to the nature of MOCVD growth where atomic hydrogen isavailable to form neutral complexes with magnesium acceptors, AlGaN:Mgfilms are insulating as grown and require thermal annealing to activatep-type conductivity. While an insulating burying layer is typicallypreferable, activated AlGaN:Mg (having p-type conductivity) is alsosuitable for burying layers if it is difficult or not possible todeposit an insulating burying layer. When buried layer 155 is p-type, ap-n junction is formed at the interface with short period superlatticen-type cladding layer 121. However, the turn-on voltage of this p-njunction is greater than the p-n junction in InGaN multiple quantum wellstructure 145. This favors the current path preferentially going throughInGaN multiple quantum well structure 145. Because no p-GaN cap isdeposited over buried layer 155, the contact of p-electrode 190 toburied layer 155 is significantly more resistive than the contact ofp-electrode 190 to p-GaN:Mg 185. This further favors current injectioninto multiple quantum well structure 145.

[0019] An n-type burying layer may also be used in order to furtherreduce optical losses because free-carrier loss is lower for n-typematerial or if it is only possible to grow n-type AlGaN material. FIG. 2shows index-guided, buried heterostructure AlGaInN laser diode structure200 with n-burying layer 255 in accordance with an embodiment of theinvention. After regrowth of n-burying layer 255, regrown burying layeris patterned by etching, typically CAIBE. In a second regrowth,n-burying layer 255 is buried with heavily p-doped GaN:Mg layer 250,having a typical doping level of approximately 10²⁰ Mg atoms/cm³, whichalso functions as the contact layer. Alternatively, burying layer 255may also be undoped. P-doped GaN:Mg layer 250 is needed to preventp-electrode 290 from contacting n-burying layer 255.

[0020]FIG. 3 shows self-aligned index-guided, buried heterostructureAlGaInN laser diode structure 300 in accordance with the invention.GaN:Si layer 315 is positioned on Al₂O₃ growth substrate 310 and in oneembodiment layer 315 may be made of AlGaN:Si. Bulk n-cladding layer 320,typically Al_(0.07)Ga_(0.93)N:Si, is positioned below the GaNn-waveguide layer (not shown in FIG. 3) at the bottom of InGaN multiplequantum well structure 345 and over n-cladding short period superlattice321. N-cladding short period superlattice 321 is typically made up ofalternating layers of AlGaN:Si and GaN:Si each with a typical thicknessof about 20 Å. Bulk n-cladding 320 prevents injection of carriers fromovergrown layer 325, typically Al_(0.07)Ga_(0.93)N:Mg to provideoptimized transverse waveguiding, into the low bandgap portion ofn-cladding short period superlattice 321. Overgrown layer 325 functionsboth as the burying layer and as the upper p-cladding layer. Hence, theoverall thickness of AlGaN in overgrown layer 325 positioned above GaNp-waveguide layer (not shown in FIG. 3) and the tunnel barrier layer,typically Al_(0.2)Ga_(0.8)N:Mg, (not shown in FIG. 3) that are locatedat the top of InGaN multiple quantum well structure 345 is on the orderof the thickness used in conventional nitride lasers. Layer 385,typically GaN:Mg, serves as a capping layer to facilitate ohmic contactto p-electrode 390. Dashed line 303 shows the location of the p-njunction in laser diode structure 300.

[0021] Overgrown layer 325 functions as both the p-cladding layer andthe burying layer to create both strong lateral current confinement andoptical confinement. The strong lateral index guiding (typically anindex step on the order of 0.1) provided by overgrown layer 325 allowslow threshold current and beam stability. Strong index-guiding allowsthe laser stripe to be made very narrow which facilitates lateral heatdissipation and lowers the required threshold current. The lateral widthof InGaN multiple quantum well structure 345 can be made very narrowbecause of the strong index guiding, typically less than 2 μm, toprovide for a low threshold current and for lateral mode discrimination.Self-aligned index-guided, buried heterostructure AlGaInN laser diodestructure 300 shown in FIG. 3 provides a greater p-contact area thanindex-guided, buried heterostructure AlGaInN laser diode structure 100shown in FIG. 1. A greater p-contact area results in less contactresistance. Lowering contact resistance reduces laser diode heatingparticularly in continuous wave operation and a wider p-contact alsoserves to better dissipate heat. Current preferentially flows throughInGaN multiple quantum well structure 345 because the p-n junctionbandgap is lowest along that portion of dashed line 303.

[0022]FIG. 4 is an expanded view of InGaN multiple quantum wellstructure 345 in FIG. 3 and shows carrier injection paths 401 and 402for self-aligned index-guided, buried heterostructure AlGaInN laserdiode structure 300. P-doped waveguide 407, typically GaN, and n-dopedwaveguide 408, typically GaN, are also shown. Dashed line 303 traces thelocation of the p-n junction. In an embodiment in accordance with theinvention operating at a wavelength of about 400 nm, InGaN multiplequantum well structure 345 has a bandgap energy of about 3.1 eV whileunderlying n-waveguide 408 has a bandgap energy of about 3.4 eV. Hence,the turn-on voltage for the p-n junction associated with InGaN multiplequantum well region 414 is lower than that of the p-n junctionassociated with n-waveguide 408 and carriers are preferentially injectedalong injection path 401 into InGaN multiple quantum well region 414when laser diode 300 is forward-biased.

[0023] The 300 meV difference between the bandgap energy of InGaNmultiple quantum well region 414 and n-waveguide 408 may in some casesbe insufficient for confining carrier injection to injection path 401and some carriers may be injected along injection path 402 across thep-n junction at the sidewalls of n-waveguide 408. Because carriersinjected across the p-n junction at the sidewalls of n-waveguide 408 donot populate the quantum wells, these carriers do not contribute tohigher optical gain and cause a higher threshold current to be required.Operation at wavelengths higher than about 400 nm such as about 430 nmwould increase the bandgap energy differential so that carrier injectionacross the p-n junction at the sidewalls of n-waveguide 408 issignificantly reduced.

[0024] Lateral injection of carriers across the p-n junction at thesidewalls of GaN n-waveguide 408 may be reduced by using an invertedasymmetric waveguide structure as shown in FIG. 5 which eliminatesn-waveguide 408. This eliminates carrier injection along injection path402 shown in FIG. 4. Tunnel barrier layer 546 lies over InGaN multiplequantum well region 514 and is typically AlGaN with an aluminum contentbetween 5 to 15 percent. P-waveguide 507, typically GaN, is located overtunnel barrier layer 546. P-cladding layer 525, typically AlGaN:Mg,covers p-waveguide 507 and buries entire laser ridge structure 511.Capping layer 585, typically GaN:Mg, provides contact to p-contact 590.

[0025] InGaN multiple quantum well region 514 is positioned on bulkn-cladding layer 520, typically AlGaN:Si. Bulk n-cladding layer 520 isplaced over short period superlattice n-cladding structure 521,typically made up of alternating layers of AlGaN:Si and GaN:Si with eachwith a typical thickness of about b 20 l Å. Bulk n-cladding layer 520blocks charge carriers from being injected from p-cladding layer 525into the typically lower bandgap GaN:Si layers of short periodsuperlattice n-cladding structure 521. Introduction of short periodsuperlattice cladding structure 521 allows cladding layers with the sameaverage aluminum content as bulk n-cladding layer 520, typically about 8percent, to be grown to a thickness of more than 1 micron whereas bulkn-cladding layer 520 is usually limited to a typical thickness of about0.5 μm before cracking occurs. Increased thickness provided by shortperiod superlattice cladding structure 521 significantly reduces leakageof the transverse optical mode and results in an improved transversefar-field pattern for laser diode structure 500. For example, a typicalleakage of about 7% may be reduced to 0.5%. The far-field beam patternapproaches a Gaussian far-field beam. N-layer 515, typically AlGaN:Si orGaN:Si, underlies short period superlattice cladding structure 521 andis placed over substrate 510, typically Al₂O₃.

[0026] Index-guided, buried heterostructure AlGaInN laser diodestructure 100 in FIG. 1 may be fabricated by first CAIBE (chemicallyassisted ion beam etch) etching through layers 185, 125, 145 and 121 toexpose n-type layer 115 for deposition of n-electrode 195. Growthrelated to GaN is disclosed in U. S. patent application Ser. No.09/288,879 entitled “STRUCTURE AND METHOD FOR ASYMMETRIC WAVEGUIDENITRIDE LASER DIODE” by Van de Walle et al. hereby incorporated byreference in its entirety. A possible issue with p-type material growthis magnesium turn on delay due to Cp₂Mg sticking to gas lines ratherthan entering the reactor. Magnesium turn on delay may be compensatedfor by pre-flowing Cp₂Mg into the reactor prior to heating and growth.The magnesium is switched to vent during the heatup and then switchedback into the reactor without turn on delay when magnesium doping isdesired.

[0027] In one embodiment in accordance with the present invention,photoresist is applied to GaN:Mg layer 185 to define the top of ridgestructure 111. However, before applying the photoresist it isadvantageous to activate GaN:Mg layer 185. The activation avoidspossible hydrogen evolution during processing which causes bubblingunder the photoresist. Activation is typically performed in one of twoways. Normal thermal activation may be used by heating to approximately850° C. for 5 minutes in a nitrogen environment. Alternatively, GaN:Mglayer 185 may be exposed to intense UV light to release the hydrogenpreventing possible thermal degradation of the surface. The photoresiststripe is lithographically patterned with the photoresist stripe alignedalong the <1100> crystallographic direction of GaN layer 185.Subsequently, the stripe is etched to produce a ridge structure 111,typically having a width from 1 to 5 μm. Ridge structure 111 is formedby CAIBE etching through layers 185, 125, 145 to short periodsuperlattice n-cladding structure 121. Note that the length axis ofridge structure 111 is oriented perpendicular to the set of {1100}planes and aligned along the <1100> crystallographic direction due tothe orientation of the photoresist stripe prior to etching. Thisorientation has been found to reduce surface pitting.

[0028] Cleaning is performed prior to epitaxial regrowth and includesphotoresist removal using a combination of dissolution in acetone andashing in an oxygen plasma. Further cleaning is performed using aquaregia then H₂SO₄:H₂O₂:H₂O mixed in the ratio 4:1:1, respectively andused as-mixed (hot). A final rinse is performed with de-ionized waterfollowed by drying in pure nitrogen.

[0029] The regrowth occurs at a stabilized temperature of 900° C. in anammonia/hydrogen gas stream. When the growth temperature has stabilizedthe reactants trimethylaluminum, trimethylgallium andbiscyclopentadienylmagnesium are introduced into the reactor. Insulatingovergrowth of burying layer 155 is accomplished by growing an undopedfilm at low temperature (T_(growth)<900° C.). Epitaxial regrowth ofburying layer 155, typically made of insulating AlN or AlGaN, isperformed to surround the ridge structure. Alternatively, a p-dopedburying layer 155, typically AlGaN:Mg may be grown. An opening is etchedusing CAIBE into burying layer 155 down to p-cap 185 to open up a narrowwindow for contacting p-cap layer 185 with p-electrode 190.

[0030] Further processing of index-guided, buried heterostructureAlGaInN laser diode structure 100 involves p-dopant activation byannealing at 850° C. for 5 minutes in a nitrogen ambient. Palladiump-contact metal deposition is evaporatively deposited and alloyed at535° C. for 5 minutes. Mirror facets (not shown) are formed by cleavingor etching. If mirror facets are etched, the etch of the first mirrorfacet is performed along with the mesa etch. N-metal deposition isperformed of Ti/Al. Finally, n-electrode 195 and p-electrode 190 aredeposited and high reflection coatings TiO₂/SiO₂ are applied to thefirst and second mirrors.

[0031] Processing for laser structure 200 is similar to that of laserstructure 100. However, in FIG. 2 n-type burying layer 255, typicallyAlGaN:Si, is regrown followed by regrowth of p-burying layer 250,typically GaN:Mg. Additionally, after n-metal deposition takes place,high temperature dielectric deposition, typically of SiN or SiO₂, isperformed over the entire surface using PECVD (plasma enhanced chemicalvapor deposition). The deposited dielectric is then patterned to createwindows for n-electrode 195 and p-electrode 190. Patterning is usedinstead of a photoresist mask because the deposition temperature for thedielectric is approximately 250° C. and photoresist is limited totemperatures below about 120° C. This makes a restricted contact windowfor p-electrode 290 contacting p-burying layer 250 to avoid currentinjection outside the laser stripe. Alternatively, ion implantation atenergies typically from about 80-120 keV may be used to create therestricted window by masking the window regions and then performing theion implantation.

[0032] Processing for self-aligned, index guided, buried heterostructureAlGaInN laser diode structures 300 (see FIG. 3) and 500 (see FIG. 5) issimilar to that for laser diode structures 100 and 200. A key differenceof the self-aligned, index guided, buried heterostructure AlGaInN laserstructures 300 and 500 is that the deposited p-doped layers 325 and 525serve both as p-cladding layers and as burying layers. Due to theself-aligned structure of laser diodes 300 and 500 there is also noetching through burying layers 325 and 525, respectively. Note thatprocessing is performed so that the length axis of both ridge structure311 (see FIG. 3) and ridge structure 511 (see FIG. 5) is aligned alongthe <1100> crystallographic direction to reduce surface pitting.

[0033] FIGS. 6-11 show processing steps for making a laser diodestructure similar to self-aligned, index guided, buried heterostructureAlGaInN laser diode structure 300. Tunnel barrier layer 646, if desired,lies between multiple quantum well region 345 and p-doped waveguide 407.

[0034]FIG. 6 shows the deposited epitaxial structure up to and throughp-doped waveguide region 407. Note that no p-cladding or capping layersare present. FIG. 7 shows the CAIBE etching of trenches 710, typicallyabout 10 μm wide, surrounding ridge 720 which is typically about 1-2 μmwide. The etching must penetrate into, but not through bulk n-claddinglayer 320. This results in an etch of about 300 nm for a typicalthickness of multiple quantum well region 345 and waveguides 407 and408. FIG. 8 shows MOCVD growth of p cladding layer 325 to a typicalthickness of about 0.5-1.0 μm. P-capping layer 385 is also grown to atypical thickness of about 0.1 μm over the structured surface. Theremaining process sequence is similar to that of conventionalridge-waveguide nitride lasers except that the ridge-etch step is notperformed.

[0035]FIG. 9 shows deposition of p-metal layer 390, typically palladiumalloy, at 535° C. for 5 minutes in a nitrogen ambient. FIG. 10 showsCAIBE etching of p-metal layer 390 and CAIBE etching to a depth of about2 μm to penetrate through n-cladding short period superlattice 321 intoGaN:Si layer 315. This etch exposes the area for n-lateral contact 1101.The first and second mirrors (not shown) are also CAIBE etched in thisstep. Liftoff metallization (typically Ti—Al) is performed for n-contactpad 395. FIG. 10 shows metallization, typically Ti—Au, to build up metalthickness on n-contact 1101 and p-contact 1102. Finally, SiO₂/TiO₂mirror coating evaporation is performed.

[0036] While the invention has been described in conjunction withspecific embodiments, it is evident to those skilled in the art thatmany alternatives, modifications, and variations will be apparent inlight of the foregoing description. Accordingly, the invention isintended to embrace all such alternatives, modifications, and variationsthat fall within the spirit and scope of the appended claims.

What is claimed is:
 1. An index guided, buried heterostructure nitridelaser diode structure comprising: a ridge structure having a first,second, and third surface, said ridge structure comprising a claddingstructure and a cladding layer with a multiple quantum well structureinterposed between said cladding structure and said cladding layer; anda burying layer overlying said first, said second and said third surfaceof said ridge structure, said burying layer having an opening to saidthird surface of said ridge structure for electrical contact.
 2. Thenitride laser diode structure of claim 1 wherein said cladding structureis a shortperiod superlattice.
 3. The nitride laser diode structure ofclaim 1 wherein said ridge structure is oriented along the <1100>crystallographic direction.
 4. The nitride laser diode structure ofclaim 1 wherein said burying layer is p-doped.
 5. The nitride laserdiode structure of claim 1 wherein a tunnel barrier layer adjoins saidmultiple quantum well structure.
 6. An index guided, buriedheterostructure nitride laser diode structure comprising: a ridgestructure having a first, a second, and a third surface, said ridgestructure comprising a cladding structure and a cladding layer with amultiple quantum well structure interposed between said claddingstructure and said cladding layer; a first burying layer overlying saidfirst, said second and said third surface of said ridge structure, saidfirst burying layer having an opening to said third surface of saidridge structure for electrical contact; and a second burying layeroverlying said first burying layer, such that said second burying layeris in contact with said third surface of said ridge structure.
 7. Thenitride laser diode structure of claim 6 wherein said first buryinglayer is n-doped.
 8. The nitride laser structure of claim 6 wherein saidsecond burying layer is comprised of magnesium doped GaN.
 9. The nitridelaser structure of claim 6 wherein said ridge structure is orientedalong the <1100> crystallographic direction.
 10. The nitride laserstructure of claim 6 wherein said cladding structure is a shortperiodsuperlattice.
 11. A method for making an index guided, buriedheterostructure nitride laser diode structure comprising the steps of:providing a ridge structure having a first, second, and third surface,said ridge structure comprising a cladding structure and a claddinglayer with a multiple quantum well structure interposed between saidcladding structure and said cladding layer; and adding a burying layeroverlying said first, said second and said third surface of said ridgestructure, said burying layer having an opening to said third surface ofsaid ridge structure for electrical contact.
 12. The method of claim 11wherein said cladding structure is a shortperiod superlattice.
 13. Themethod of claim 11 wherein said ridge structure is oriented along the<1100> crystallographic direction.
 14. The method of claim 11 whereinsaid burying layer is p-doped.
 15. The method of claim 11 wherein atunnel barrier layer adjoins said multiple quantum well structure.
 16. Amethod for making an index guided, buried heterostructure nitride laserdiode structure comprising the steps of: providing a ridge structurehaving a first, a second, and a third surface, said ridge structurecomprising a cladding structure and a cladding layer with a multiplequantum well structure interposed between said cladding structure andsaid cladding layer; adding a first burying layer overlying said first,said second and said third surface of said ridge structure, said firstburying layer having an opening to said third surface of said ridgestructure for electrical contact; and adding a second burying layeroverlying said first burying layer, such that said second burying layeris in contact with said third surface of said ridge structure.
 17. Themethod of claim 16 wherein said first burying layer is n-doped.
 18. Themethod of claim 16 wherein said second burying layer is comprised ofmagnesium doped GaN.
 19. The method of claim 16 wherein said ridgestructure is oriented along the <1100> crystallographic direction. 20.The method of claim 16 wherein said cladding structure is a shortperiodsuperlattice.